In most telecommunications networks (such as GSM, UMTS or LTE), a functionality exists for identifying and locating mobile terminals on a regular basis in order to route calls and data to them.
An identifier called the IMSI (International Mobile Subscriber Identity) is stored in each SIM (or USIM) card. This identifier identifies a subscriber associated with the SIM card in any telecommunications network in any geographical area. For example, the IMSI allows identifying the country of origin of the subscriber, the network he subscribes to, as well as his identity. Because the SIM card is inserted in a mobile terminal, the IMSI identifies the terminal.
To ensure a certain degree of anonymity for the user of the mobile terminal, the IMSI number is exchanged in a very limited manner between the mobile terminal and the entities that manage the network.
Each time the mobile terminal is turned on, the IMSI is exchanged only once, then the network sends the mobile terminal a temporary identification number or TMSI (Temporary Mobile Subscriber Identity). This TMSI number is then used in exchanges between the mobile terminal and the network until the mobile terminal is turned off or leaves the network.
The TMSI is also associated with particular areas in the network. As the network is subdivided into cells, a TMSI is delivered to the mobile terminal each time it enters a new cell. Such terminal location updating is, for example, required by the 3GPP standard (3GPP 24.008).
At all times the mobile terminal is therefore identified by a unique TMSI uniquely associated with a network cell.
The prior art process of allocating the TMSI is illustrated in FIG. 1.
In FIG. 1, two cells CELL1 and CELL2 of a telecommunications network are represented. Found in each of these cells is a base station BTS1 and BTS2 for communicating with the mobile terminals located in these cells. The base stations are controlled by base station controllers BSC1 and BSC2, which are connected to a mobile communications control center (MSC for Mobile Switching Center), which is itself connected to VLR and HLR registers in order to manage the mobile terminals subscribed to the network and those in transit within the network.
The terminal (UE for User Equipment) travels through cells CELL1 and CELL2 between positions a, b, c, and d. Positions a and c are within the coverage of CELL1 and positions b and d are within the coverage of CELL2.
When the mobile terminal UE is turned on, it transmits its IMSI number to the MSC via the base station BTS1. In response, the MSC assigns it a temporary number TMSI1. This number is then used by the mobile terminal and the network for all data exchanges as long as the terminal does not leave CELL1.
In order to travel to position b, the mobile terminal leaves CELL1 and arrives in CELL2. As it enters this new cell, the mobile terminal transmits a location update request. For this purpose it transmits its number TMSI1 to the MSC via the base station BTS2. In response, the mobile terminal receives a new identification number TMSI2. This new number identifies it in CELL2, and this number is used for all communications between the mobile terminal and the network as long as it does not leave CELL2.
In order to travel to position c, the mobile terminal leaves CELL2 and returns to CELL1. When it arrives in CELL1, it sends a location update request and transmits its number TMSI2 to the MSC via the base station BTS1. In response, it receives a new identification number TMSI3. It is this new number that is used by the mobile terminal as long as it does not leave CELL1.
Lastly, in order to move to position d, the mobile terminal leaves CELL1 and arrives in CELL2. It transmits a new location update request and for this purpose sends its identification number TMSI3 to the MSC via the base station BTS2. In response, it receives a new identification number TMSI4 which it uses as long as it does not leave CELL2.
Each time it enters a new cell, the mobile terminal exchanges data in this manner with the network management units in order to receive a new identification number.
The result is a volume of exchanged data which monopolizes part of the bandwidth used for communications. In addition, this requires the implementation of special mechanisms within the network in order to manage the identification numbers.
On the mobile terminal side, the process requires the use of special resources which consume some of the battery power of the mobile terminal.